Sifter



July 26, 1960 E. F. FORD SIFTER Filed March 10, 1959 2 Sheets-Sheet 1 INVENTOR.

EVERETT F. FORD 7M4; (fa/1W4 ATTORNEY July 26, 1960 E. F. FQRD 2,946,441

SIFTER Filed March 10, 1959 2 Sheets-Sheet 2 EVERETT E FORD ATTORNEY United States Patent SIFTER Everett F. Ford, Des Moines, Iowa, assignor to General Mills, Inc., a corporation of Delaware Filed Mar. 10, 1959, Ser. No. 798,465

' 8 Claims. 01. 209-344 tory or reciprocating sifter for classifying finely ground or powdered materials, such as flour milling stocks.

Sifting devices are known in the prior art in which stock is conveyed across a screen or sifting surface by suitable recipro'cation or vibration of the sifting unit. During such movement, the stock is classified into separate fractions of different particle size. In such sifters the sifting surface is often enclosed in an outer box or frame to prevent escape of fine particles of stock. Both the box and enclosed sifting surface are then subjected to rapid vibratory reciprocatio'n to convey the material across the sifting surface and obtain the desired separation.

One problem encountered in the application of such a sifter to finely ground or powdered materials is that of irregularity or surging in the sifting and conveying action of the stock on the surface. Such surging has been found to involve the adverse effect of variations in the instantaneous dynamic air pressures above and below the sitting surface. Such pressure changes may cause the stock to be held fast to the screen when it should be free to slide, and conversely to be blown upwardly away from the screen when it should be in contact with-it. The problem is particularly encountered at rapid frequencies of reciprocation, e.g. of the order of six strokes per second.

With the problems of the prior art in view, it is one object of the present invention to provide .classifying apparatus of the reciprocating type with improved means for minimizing adverse dynamic pressure effects above and below the screen.

Another object is to provide an improved reciprocating or vibratory sifter having novel means for equalizing dynamic pressures above and below the sifting surface.

. A further object is a reciprocating vibratory sifter provided with a novel arrangement of partition means at the edges of the sifting surface,.said partition means providing pressure equalizing passages the sifter between the chambers above and below the sifting surface. l. Other objects and advantages will be apparent from the following specification, in which certain preferred embodiments of the invention are described.

.In general, the above objectives are accomplished-according to the present invention by the use of at least one longitudinally extending partition which is spaced inwardly from the side wall of the sifter box and serves as a guide or boundary atthe edge of the stock which is being conveyed along the sifting surface. The construction and location of the partition provides an equalizing air passage from the region of the upper chamber 2,946,441 Patented July 26, 1960 prevent undesired movement of stock from the upper to the lower chamber through the equalizing air passage.

In the drawings which form a part of this application,

Fig. 1 is a side view, partly in section, of a vibratory reciprocating sifter embodying features of the present invention.

Fig. 2 is a sectional view on the line 2--2 of Fig. 1.

Fig. 3 is a partial perspective view of the left hand side (with reference to the direction of movement of stock) of the sifter of Fig. l.

Fig. 4 is a view similar to Fig. 2 of another embodiment ofthe invention, and

Fig. 5 is a partial view, similar to part of Fig. 2, of still another embodiment.

As shown in Figs. 1-3, the sifter includes a body por tion 10 made up of bottom wall 12, top wall 14, end walls 16 and 18, and side walls 20 and 22. These walls provide a substantially closed chamber designed for longitudinal reciprocation. For this purpose, the sifter body is suspended by swinging hanger members 24. The lower ends of hangers '24 are pivotally connected at 26 to the sifter body. The upper ends of hangers 24 are pivotally supported at 28 on a suitable frame or wall member 30.

The sifter body 10 is reciprocated by a driving rod 32, one end of which is pivotally connected at 34 to the sifter body and the other end of which is driven by an eccentric 36 on shaft 37. The driving shaft 37 is in turn driven by any suitable power source, such as motor 40, provided with a manually operable speed adjustment handle 42. Thus the sifter can be reciprocated longitudinally at a desired frequency and amplitude to achieve the desired conveying movement and classification.

above the stock on the sifting surface, through the space 4 between the partition and side wall of the sifter, and thence to the lower chamber below the classifying surface. In one form of the invention, a portion of the classifying surface or screen is extended laterally beyond the partition member across the intermediate space to The actual sifting is done by a sifting surface or screen of known type such as a metal or cloth screen indicated generally at 44. This classifying surface has an inlet end area 46, a discharge end area 48, and an intermediate sifting area 50. The shifting surface or member 44 extends longitudinally across the major portion of the box 10 and thus effectively divides the box into upper and lower chambers 52 and 54.

An inlet opening 56 is provided at the inlet end of the upper chamber 52 and is defined by an inlet spout 58, connected by a flexible coupling '60 to an appropriate supply hopper or prior processing machine 62. Stock which enters inlet 56 is then conveyed longitudinally along the sifting surface 44, as shown at 63, and crosses the sifting area 50 to achieve the desired separation. Particles too coarse to pass through the sieve proceed to the discharge end 48 of the screen and are discharged from the upper chamber through an outlet 64 suitably connected to a subsequent processing machine, receptacle or conveyor 66.

The finer particles of stock which pass through the openings in the shifting area 50 are shown at 67 and are conveyed along the bottom wall 12-or an equivalent conveying pan to an outlet 68 at the discharge end of lower chamber 54. Outlet 68 in turn is connected to any suitable subsequent apparatus or receptacle 70. The invention isparticularly designed for sifting flour art as a 12 XX silk screen or a similarly fine wire screen.

As shown in Fig. 2, the screen is supported at the desired height above bottom wall '12 by longitudinal side supports or guides 72. One or more intermediate longitudinal supports 74 will also be used with screens which are wide enough to sag undesirably between supports .72.

The screen may be tensioned at thev desired degree by known arrangements. For example, the lateralledges 76 of the screen may be engaged by clamping members 78 which can be tensioned laterally by adjusting nuts 80 engaging the threaded ends of the clamping members which pro'ject through side walls 20 and 22.

To hold the screen down on the supporting members 72, a sealing strip 82 is used at each edge of the upper chamber. A layer of felt or plush 84 may be secured to the edges of strips 82, as well as the remaining partition and support members where such members engage the screen itself, to prevent abrasion and facilitate ad justment of the screen tension. Sealing strips 82 may be secured as at 86 to the top wall 14 of the sitter chamber. To prevent undesired upward vibration of the intermediate portions of screen 44, additional upper guides or partitions 88 extend longitudinally of the sifter and upper chamber above each of members 74. Guides 88 are also secured to the top wall 14 as at 96.

For convenience in assembly and maintenance, the top wall 14 may be provided as a removable cover socured by suitable fastening members 92.

In actual operation, a sitter as described up to this point often exhibits uneven and erratic movements of the stock, which has been termed surging." For example, the conveying movement of stock at the head or receiving end has been observed to slow up and even back up toward the inlet, while stock at the tail end moves even faster off the screen and leaves a gap or bare area on the screen. A thick pile of stock then accumulates at the head or inlet end and gradually expands along the screen, but without apparent conveying action. As the stock finally expands toward the discharge end and covers the screen, the conveying movement of the layer may begin again. Stock near the tail or discharge end moves oil more rapidly, until the layer again seems to break in the center screen area, with the portion nearest the inlet backing up toward the inlet and the other portion moving more rapidly to the outlet. Such surging impairs the sifting action, since some stock is held and sifted too long at the head end, while other stock is discharged too soon with inadequate sifting.

It is believed that the inertia of the fluctuating air columns within the upper and lower chambers of the sitter tends to affect the sifting and conveying operation by creating pressure difierentials above and below the sizing screen 50. These pressure differentials appear to work in opposition to the desired sifting and conveying action.

For example the construction of the sitter is such that the ends of the upper and lower chambers are not symmetrical or balanced in their effects on the enclosed air columns. Thus the area at the inlet end of the lowermost chamber 54 is essentially closed by the lower portion of head end wall 16. On the other hand, the inlet 56 through which the material enters the sifter body provides a sizeable opening for the corresponding head end of upper chamber 52 above the screen 50. During sifting as noted above, the differences in relative freedom of movement of the air in these compartments (excluding for the moment the features to be described below) will cause pressure differentials which can give rise to the undesirable results mentioned previously.

Thus air or fluid columns in the compartments 52 and 54 above and below the screen 50 have an inertia which tends to hold the fluid column stationary as the sitter vibrates. If the sifter 10 is moving to the right, however, in Fig. l, the tendency of the air to remain in its original position, and the movement of end 'Wall 16 to the right, will produce a region of higher density or highor pressure at the head end of chamber 54 during such movement. Conversely, when the sifter moves to the left, the end wall 16 tends to move away from the enclosed column of air in chamber v54 to produce an area of lesser density or lower pressure at the head end of chamber 54. These pressure variations within the sifter 10 are for practical purposes termed air slugging or dynamic air pressures.

tion of end wall 16 and sitter 10 to the right.

The eifect of air-slugging is somewhat dilferent above the screen 50, however, since the inlet 56 will permit the escape of part of the air'jcolumn from the head end of upper chamber 52 during movement of the upper por- Likewise the inlet 56 will admit further air as the end wall -16 and sifter 10 move to the left. Thus the inertia of the body of air above the screen 50 will not cause airslugging or dynamic pressure of the same magnitude as that below the screen. Hence the higher pressure at chamber 54 tends to lift the material being conveyed away from the screen 50 just when it should be in contact with the sifting surface in order to obtain the desired conveying and sifting movement to the right. Likewise the lower pressure at chamber 54 during the movement of the sifter ll) to the left tends to draw the material conveyed down against the screen 50 and move the material to the left, just when it should remain free of the beneath the outlet end of screen 50.

screen in. order to obtain relative movement toward the tail end of the sifter.

At the tail or discharge ends of the respective upper and lower chambers 52 and 54, the effects of dynamic pressures appear to increase the conveying action. Here, when the upper portion of tail end wall 18' moves to the right, as seen in Fig. 1, it tends to create an area of low pressure above the tail end of screen 50.

While the lower portion of end wall 18 may have a similar effect in the region of outlet 64, an even more important factor in this area is the presence of the usual frame member 71 which supports the end of screen 50. This member acts as a wall portion immediately below the tail end of the screen. As it moves to the right, it further decreases the dynamic pressure The net result is a pressure differential in which the lower pressure below the tail end of the screen holds stock on the screen as the screen moves to the right.

Conversely, the increased dynamic pressures due to movement of wall 18 and frame 71 to the left (arrow 60) result in a pressure differential which pushes the stock upwardly away from the screen on this return stroke; The net effect is to increase the conveying efiiect and hurry the material off the discharge end of the screen.

According to the present invention, a novel arrangement is provided to equalize the freedom of movement of the air and the instantaneous dynamic air pressures above and below the sifting area 50. For this purpose, the sifter includes at least one longitudinally extending partition means or member 94, which is spaced inwardly from the side wall of the sifter. Preferably two such members are used, one at each side.

The lower portion 96 of each member 94 is imperforate and extends from the sifting surface upwardly to a level above the normal stock level expected in operation of the device. The upper edges of partitions 96 may, for convenience, be secured at 98 to the removable top wall 14.

According to the invention, the construction and location of this partition member provides an equalizing air passage 100 from the upper portion of chamber 52, i.e. above the level of stock 63, to the lower chamber 54. The upper portion of this air passage is preferably defined by a series of openings or cut-away portions 101 spaced along the upper edge of partition 94. These openings 101 thus connect the upper chamber 52 to the space 102 between the partition 94 and its nearest side wall 22. This space, in turn, communicates with the lower chamber 54. Thus, air pressures above and below the screen area 50 can be equalized instantaneously as a result of the free communication between the two chambers along the paths mentioned.

To avoid undesired shaking of any of the coarser portions of stock 63 through the opening 100 and thence to the lower chamber, the form of invention shown in Figs.

1-3 also includes a barrier or screen for the stock. As illustrated, such a barrier is preferably provided by extending the screen portion of the sifting surface itself laterally beyond partition 94 and across space 102 as shown at 104. Screen portion 104 thus normally performs no classifying function. Should any of stock 63 be accidentally through into space 102, however, screen portion 104 would permit the finer particles to pass to the lower chamber 54, while the coarser particles would be conveyed longitudinally through the sifter to the discharge outlet 64, just as in the case of the main stock portion 63 between partitions 94.

To further insure substantially instantaneous equalization of dynamic air pressures above and below the screen, all longitudinally extending auxiliary supports 74 and 88 are provided with lateral openings 108 and 106, respectively. In other words, supports 74 and 88 are substantially cut away as much as is practical without interfering with their support function, to provide free lateral communication for the air in the upper and lower chambers and thus insure that pressure conditions above and below the screen will be as uniform as possible throughout the lateral width of the screen. In some cases, of course, if the screen is sufliciently narrow, so that vertical vibrations of the intermediate screen area will not occur under normal stock loads, auxiliary supports 74 and 88 could be omitted. In any event, the essential features of the present invention include the provision of the longitudinal partition spaced inwardly from a side wall of the sifter, thus providing a normal boundary or guide for the edge of the sifting area; said partition also providing, in combination with the space between the partition and side wall, an effective equalizing air passage between the upper and lower chambers.

Fig. 4 illustrates a modified embodiment of the invention in which the entire width of the screen member 50 is used for conveying and classifying the stock. Here the essential elements of the sifter box, including top, bottom, side and end walls, supports and method of vibration are essentially the same as in the device of Figs. 1-3.

In this case, however, the equalizing air passage is pro vided by using as a partition and guide member the sealing or filling strips 110, which are designed to engage the top of the screen at its edges, adjacent clamps 78. Here the upper portion of guide member 110 above the expected stock level is cut away as much as practical at112 and is dimensioned and located to provide a space 114 between the partition 110 and side wall. The lower side supports 72 are also cut away to provide openings 116 therethrough. Thus, an equalizing air passage is provided from the upper chamber 52, by way of the openings 112 in partition 110, and thence through the space 114 between the outer edge of the screen and side wall of the box, then through openings 116 in the support 72 to the lower chamber 54. All intermediate partitions and supports 74 and 88 are again provided with openings 108 and 106.

In this embodiment, the entire area of the classifying screen is available for sifting and none of it is used as a barrier for the equalizing air passage at 114. By the same token, the entire cross section of the air passage is available for pressure equalization and none of it is blocked by the solid portions of the screen mesh. Since the openings in a screen constitute only a part of the total screen area, the total air passage cross section can be smaller if the screen is omitted, as shown in Fig. 4, in cases where the type of stock, conditions of vibration, and height of openings 112 above the stock are such as to minimize the possibility of undesired passage of coarser stock from the upper to the lower chamber by way of the equalizing air passage.

As one example of operating dimensions for such a device, a sifter of the type shown in Figs. 1-3 was constructed with side walls feet long and 8 inches high. The total width between side walls was 40 inches. A sifting screen was supported within the box approximately 3 /2 inches above the bottom wall. The extending screen portions 104 across the air spaces at the edges of the sifter were each substantially 2 /2 inches wide, and the effective sifting area was approximately 30 inches wide. Each side partition 94 had a total of 2 openings 100, each having an average vertical height of inch and a longitudinal dimension of 26 /2 inches.

In another example of similar sifter box dimensions, where the space between the side walls and partition members was left open with no screen area across it, i.e. a sitter of the type of construction in Fig. 4, a width of substantially one inch for this airspace 114 at each edge of the sifter was considered adequate. While the relative dimensions of open vs. screened equalizing air passages will depend on the percentage of open area of the given screen, with flour cloths of fine wire mesh a screen width of 2% inches thus was considered equivalent to an open area of 1 inch width.

For convenience, the invention has been illustrated in reciprocating sifters of the single screen type. The invention can also be used where a plurality of such sifting screen units are superimposed with intermediate imperforate conveying pans, thus providing pneumatically separate sifting chambers within a single sif-ter box, which are vibrated as a unit, as in Fig. 5. The parts and operation of each separate superimposed unit are essentially the same as in Fig. 4.

According to the foregoing description, this invention contemplates the use of longitudinal partitions at the edges of the sifting area, spaced from the side walls and providing equalizing air passages within the sifter box between the chambers or zones above and below the screen. -In one form described, portions of the sifting screen are kept free of stock to equalize air pressures above and below the screen. The specification includes a description of the principles which are believed to be involved in the present invention, together with some of the ways in which the invention can be practiced.

I claim:

1. In a reciprocating classifying unit comprising a closed body portion having top, bottom, side and end walls, a classifying surface located within and extending across said body portion at a level between the top and bottom walls and thereby dividing the body portion into upper and lower chambers, said classifying surface having a receiving end, a discharge end, and an intermediate sifting area, an inlet for stock at the receiving end in the upper chamber, an outlet for stock at the discharge end in each chamber, and means for reciprocating said body and classifying surface longitudinally and thereby conveying stock from said receiving end to said discharge end while classifying the stock by said sifting area, the improvement comprising partition means extending longitudinally along atleast one side of said sifting area generally parallel to and spaced from the nearest side wall, said partition means having an imperforate guide portion extending upwardly from the classifying surface above the expected depth of stock to be classified, and having means providing an equalizing air passage of substantial cross section extending from the upper chamber above the stock to the space between the partition and side wall and thence to the lower chamber.

2. A classifier according to claim 1 having a screen portion having perforations at least as fine as said sifting area and extending across said equalizing air passage, thereby permitting free flow of air between the upper and lower chambers and blocking undesired movement of stock therebetween.

3. A classifier according to claim 1 in which said sifting surface consists of a perforated screen member which extends beyond said partition member across the space between said partition and said nearest side Wall, thereby permitting free flow of air between the upper and lower chambers and blocking undesired movement of stock therebetween.

-4. A classifier according to claim 1 in which said top 7 'wall includes a removable cover section and said partition means is attached to and'removable with said cover.

5. In a sitter comprising a closed body portion having top, bottom, 'side and end walls, a classifying'surface located withiniand extending across said body portion a t-a level between the top and bottom Walls and thereby dividing the body portion into upper and lower chambers, said classifying surface having a receiving end, a discharge end, and an intermediate sifting area, an inlet for stock at the receiving end in the upper chamber, an outlet for stock at the discharge end in each chamber, and means for reciprocating said body and classifying surface longitudinally and thereby conveying stock from said receiving end to said discharge end while classifying the stock by said sifting area, the improvement comprising a pair 'of'partitien members, one at each side of the sifting area, each partition extending longitudinally and spaced inwardly from and essentially parallel to the nearest side wall thereby providing an air space between the partition and side wall, each partition having a lower imperforate portion engaging the top of the sifting surface and extending upwardly therefrom to a point above the expected level of stock to be sifted, and means providing air passages extending from said upper chamber over the lower portion of each partition to said air space and thence through said air space to said lower chamber.

6. In a sifter comprising a closed body portion having top, bottom, side and end walls, a'classifying surface located within and extending across said body portion at a level between the top and bottom walls and thereby dividing the body portion into upper and lower chambers,

said classifying surface having a receiving end, a discharge end, and an intermediate sifting area, an inlet for stock at the receiving end in the upper chamber, an outlet for stock at the discharge end in each chamber,

and means for reciprocating said body and classifying surfacelongitudinallyand thereby conveying stock from said receiving end to said discharge end while classifying the stock by said sifting area, the improvement comprising, in combination, means for guiding the stock longitudinally across said sitting area in spaced relation to at least one of said side walls, and means providing an air passage extending from said upper chamber to said lower chamber between said guiding means and said one side wall.

7. A sifter according to claim 6 including screening means extending entirely across said air passage and having openings at least as small as those in said sifting area, :thereby preventing undesired movement of coarser stock from the upper to the lower chamber.

8. In a sifting apparatus which comprises a sifting screen for classifying relatively fine particles of material, means for producing sifting movement of said screen in the presence of a fluid medium, and body means enclosing and moving with said screen and forming opposed zones at opposite faces of the sifting screen in which zones the fluid medium may have dilferent degrees of movement in response to said sifting movement, the improvement comprising means for equalizing the resulting fluid pressures at the opposite faces of said sifting screen in said zones, said equalizing means including means guiding the stock along the screen and maintaining a portion of said screen clear of said stock thereby providing an equalizing air passage between said opposed zones through said clear screen portion.

References Cited in the file of this patent UNITED STATES PATENTS 1,575,457 Stebbins Mar. 2, 1926 m as 

